Lamb wave type frequency device and method thereof

ABSTRACT

A Lamb wave type high frequency device comprises: a piezoelectric substrate; an interdigital transducer (IDT) electrode formed on a first main surface of the piezoelectric substrate; a reinforcing substrate connected to a second main surface of the piezoelectric substrate; a space portion formed in one of the piezoelectric substrate and the reinforcing substrate, an area of the space portion being larger than a region in which a Lamb wave is propagated; and a connecting surface formed in a periphery of the space portion.

This is a Division of application Ser. No. 11/668,220 filed Jan. 29,2007. The disclosure of the prior application is hereby incorporated byreference herein in its entirety.

BACKGROUND

1. Technical Field

The present invention relates to a Lamb wave type high frequency deviceand a method thereof. In particular, it relates to a Lamb wave type highfrequency device in which a piezoelectric substrate having an IDTelectrode is connected with a reinforced substrate and a method ofmanufacturing it.

2. Related Art

As high frequency resonators, surface acoustic wave elements using aRayleigh wave, a Leaky wave and an SH wave and Lamb wave elements usinga bulk wave such as a Lamb wave are conventionally known.

For example, a Rayleigh wave type surface acoustic wave element isknown. In the element, an IDT electrode is formed in the Z′-axisdirection on the surface of a quartz substrate called as ST cut quartz.

Such element is disclosed in “Analysis of frequency and temperaturecharacteristics of surface acoustic waves by using an infinite elementmethod” written by Shigeo Karma in pages 37 to 42 of the technicalreport of IEICE, US 99-20(199-06), in Shingaku Giho.

Further, an SH wave type surface acoustic wave element is also known,(see JP-A-10-233645, page 3 to page 6, FIG. 1). This device propagates atransverse wave in which the propagation direction of a surface acousticwave is shifted by 90 degrees from a STW cut quartz, namely a ST cutquartz.

Further, a Lamb wave element using a bulk wave (volume wave) propagatingwith repeating reflection at the upper and lower surface of apiezoelectric substrate instead of a surface acoustic wave is alsoknown. The element is particularly used for higher frequency since itsphase velocity is larger compared to that of the surface acoustic wave(see JP-A-2003-258596 and “A substrate for a Lamb wave type surfaceacoustic wave element” written by Yasuhiko Nakagawa, Masayuki Momose andShouji Kakio in page 93 to 96 of the 33^(rd) EM symposium 2004.)

Further, in a Lamb wave type high frequency resonator using the Lambwave element, a Lamb wave is efficiently excited by using an AT cutquartz substrate as a piezoelectric substrate and setting therelationship between the thickness of the quartz substrate H and thewavelength of the Lamb wave λ to be 0<2H/λ≦10.

In the above disclosures (patent documents and non patent documents),however, the thickness of the piezoelectric substrate is from several μmto several tens μm, thereby handling the substrate is difficult since itis easily broken. In particular, as shown in JP-A-2003-258596, thethickness of the piezoelectric substrate has to be several wavelengthsin order to excite a Lamb wave, though a Lamb wave type high frequencydevice can realize higher frequency compared to a case using a surfaceacoustic wave. Hence, such device is hardly handled and easily brokencompared to the surface acoustic device, further, its yield is lowered.

SUMMARY

An advantage of the present invention is to provide a Lamb wave typehigh frequency device realizing a stable resonance property and anenhanced structural strength and a method of manufacturing it with animproved yield in which the device is uneasily broken.

According to a first aspect of the invention, a Lamb wave type highfrequency device includes: a piezoelectric substrate; an IDT electrodeformed on a first main surface of the piezoelectric substrate; areinforcing substrate connected to a second main surface of thepiezoelectric substrate; a space portion formed on the piezoelectricsubstrate or the reinforcing substrate, an area of the space portionbeing larger than a region in which a Lamb wave is propagated; and aconnecting surface formed in the periphery of the space portion.

In the above aspect, the reinforcing substrate is provided and connectedto the piezoelectric substrate, which is thin and easily broken. Thestructure realizes a Lamb type high frequency device of which astructural strength is enhanced, uneasily broken but easily handled.

Further, in the aspect, there is the space portion formed in thepiezoelectric substrate or the reinforcing substrate and having an areathat is larger than a region in which a Lamb wave is propagated. Thestructure realizes a Lamb type high frequency device that excludesenergy loss of a Lamb wave excitation at the connecting surface withmaintaining a structural strength, resulting in the stable resonanceproperty having high excitation efficiency.

Further, it is preferable that the space portion includes a boxlikerecess formed in the piezoelectric substrate or the reinforcingsubstrate.

This structure easily forms the above space portion by connecting thepiezoelectric substrate and the reinforcing substrate. The space portionincludes the boxlike recess. Hence, all circumferential areassurrounding the recess (namely the space portion) are connected, makinga Lamb type high frequency device have sufficient structural strength,even if the thickness of the piezoelectric substrate, which thickness isrequired as an exciting condition, is several μm.

Further, the recess is boxlike, making the space portion possible to beair-tightly sealed, namely as a vacuum. Hence, the structure excludesenergy loss from the interface at the backside (another main surface) ofa piezoelectric substrate that reflects a Lamb wave.

Further, it is preferable that the space portion includes a groove-likerecess formed in the piezoelectric substrate or the reinforcingsubstrate.

The cross section of the groove-like recess is an almost square C shapein which opposed side surfaces of the piezoelectric substrate or thereinforcing substrate are opened, and two circumferential directions ofthe recess are reinforced. A sacrificed layer is formed in the recessand then the sacrificed layer is easily removed from the opening of theside surface after the IDT electrode is formed. Details are describedlater.

In addition, it is preferable that the piezoelectric substrate be aquartz substrate.

Using a quartz substrate as the piezoelectric substrate realizes atemperature characteristic that is superior to that of the conventionalSTW cut quartz substrate and ST cut quartz substrate.

According to a second aspect of the invention, a Lamb wave type highfrequency device includes: a piezoelectric substrate; an IDT electrodeformed on a first main surface of the piezoelectric substrate; areinforcing substrate connected to a second main surface of thepiezoelectric substrate; a space portion formed in the piezoelectricsubstrate or the reinforcing substrate, an area of the space portionbeing larger than a region in which a Lamb wave is propagated; aconnecting surface formed in a periphery of the space portion; and apackage having a case and a lid and air-tightly housing thepiezoelectric substrate and the reinforcing substrate.

In this aspect, the piezoelectric substrate reinforced by thereinforcing substrate is encapsulated by the package, protecting thepiezoelectric substrate from the outer environments.

Further, exciting characteristics are remarkably deteriorated when theIDT electrode is hurt. But the above structure protects the IDTelectrode after packaging since the IDT is free from contact within thepackage.

Further, the package is air-tightly sealed, maintaining inside thepackage as a vacuum and restraining energy loss at the IDT electrode.Further, the space portion can be maintained as a vacuum by vacuumingthe package at only one time, even if the space portion is a groove-likerecess having an opening at the side surface.

Further, it is preferable that the connecting electrode formed withinthe case be connected with a plurality of pads formed at least on a busbar included in the IDT electrode.

Providing the pads on the bus bar does not affect the excitation. Hence,the pads provided on the bus bar can electrically connect the connectingelectrode and fix the piezoelectric substrate and the reinforcingsubstrate to the package.

The connection using the pad is called as a flip chip mounting. Usingthe flip chip mounting realizes both fixing and connectionsimultaneously, downsizing the thickness and the area for mounting, andmaking the device a low height and miniaturized compared to the wirebonding conventionally used to mount surface acoustic wave elements.

A third aspect of the invention is a method for manufacturing the Lambwave type high frequency device having the first aspect. Namely thedevice includes: a piezoelectric substrate; an IDT electrode formed on afirst main surface of the piezoelectric substrate; a reinforcingsubstrate connected to a second main surface of the piezoelectricsubstrate; a space portion formed in the piezoelectric substrate or thereinforcing substrate, an area of the space portion being larger than aregion in which a Lamb wave is propagated; and a connecting surfaceformed in a periphery of the space portion. The method comprises:forming a groove-like recess corresponding to the space portion providedin either a thick plate of the piezoelectric substrate or thereinforcing substrate; forming a sacrificed layer in the recess;connecting the thick plate of the piezoelectric substrate with thereinforcing substrate; polishing the thick plate of the piezoelectricsubstrate to a given thickness after the connection; and forming the IDTelectrode and removing the sacrificed layer after the polishing.

In the third aspect, the piezoelectric substrate is connected to thereinforcing substrate as the thick plate after the sacrificed layer isformed in the recess, and the substrate is polished to a giventhickness. Further, the sacrificed layer is removed after the IDTelectrode is fowled so as to form the above mentioned space portion.Hence, the sacrificed layer is maintained until just before the Lambwave type high frequency device is completed. This process can reducethe break of the piezoelectric substrate, improving the yield of it.

Further, forming the groove-like recess easily removes the sacrificedlayer from the opening of the both ends of the groove.

A fourth aspect of the invention is a method for manufacturing the Lambwave type high frequency device having the first aspect. Namely, thedevice includes: a piezoelectric substrate; an IDT electrode formed on afirst main surface of the piezoelectric substrate; a reinforcingsubstrate connected to a second main surface of the piezoelectricsubstrate; a space portion formed in the piezoelectric substrate or thereinforcing substrate, an area of the space portion being larger than aregion in which a Lamb wave is propagated; and a connecting surfaceformed in a periphery of the space portion. The method comprises:forming a boxlike recess corresponding to the space portion in thereinforcing substrate; forming a penetration hole in the bottom of therecess; forming a sacrificed layer in the recess; connecting a thickplate of the piezoelectric substrate with the reinforcing substrate;polishing the thick plate of the piezoelectric substrate to a giventhickness after the connection; and forming the IDT electrode andremoving the sacrificed layer after the polishing.

In these processes, forming the penetration hole connected to the recesscan remove the sacrificed layer using the penetration hole even if therecess portion is boxlike and the sacrificed layer is formed within therecess.

It is preferable that the piezoelectric substrate be made of a quartzsubstrate and the scarified layer be made of a material etched by anetchant, which is different from another etchant etching thepiezoelectric substrate.

Here, the above material etched by an etchant means a material such aszinc oxide (ZnO) and aluminum nitride (AlN). The sacrificed layer madeof one of such materials is resolved by the etchant when the layer isremoved with etching, but the piezoelectric substrate is not resolved byit.

In this process, the interface between sacrificed layer and thepiezoelectric substrate is not etched by the etchant when the sacrificedlayer is removed, attaining superior resonance characteristics when aLamb wave is reflected at the both front and back surfaces of thepiezoelectric substrate.

Further, it is preferable that the above fourth aspect of the inventionfurther comprise; forming the sacrificed layer with SiO₂, forming aprotection layer against etching on the second main surface of thepiezoelectric substrate made of a quartz substrate, and removing theprotection layer in the range which is lager than an area of the Lambwave propagated after removing the sacrificed layer.

The structural body composed of the piezoelectric substrate and thereinforcing substrate as an oscillating body is called as micro electromechanical systems (MEMS). In MEMS, it is well known that the sacrificedlayer is made of SiO₂. The etchant for the quartz substrate, however, isthe same for SiO₂ of the sacrificed layer, etching other main surface(the back surface) contacted with the sacrificed layer of the quartzsubstrate in the process of removing the sacrificed layer. The Lamb wavetype high frequency device uses a bulk wave, making the back surface ofthe device affect the oscillation characteristics. Hence, the protectionlayer against etching at the back surface of the quartz substrateprotects the quartz substrate from an etchant.

A fifth aspect of the invention is a method for manufacturing the Lambwave type high frequency device having the first aspect. Namely, thedevice includes: a piezoelectric substrate; an IDT electrode formed on afirst main surface of the piezoelectric substrate; a reinforcingsubstrate connected to a second main surface of the piezoelectricsubstrate; a space portion formed in the piezoelectric substrate or thereinforcing substrate, an area of the space portion being larger than aregion in which a Lamb wave is propagated; and a connecting surfaceformed in a periphery of the space portion. The method comprises:forming a recess corresponding to the space portion provided in either athick plate of the piezoelectric substrate or the reinforcing substrate,connecting the thick plate of the piezoelectric substrate with thereinforcing substrate; filling a thermosetting resin in the spaceportion to be a sacrificed layer and curing the thermosetting resinafter the connecting; polishing the thick plate of the piezoelectricsubstrate to a given thickness after the curing; and forming the IDTelectrode and removing the sacrificed layer after the polishing.

Using a thermosetting resin to be the sacrificed layer can fill theresin in the space portion after connecting the thick plate of thepiezoelectric substrate with the reinforcing substrate. This processdoes not need expensive manufacturing facilities such as depositionapparatuses, chemical vapor deposition (CVD) apparatuses and the like.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described with reference to the accompanyingdrawings, wherein like numbers reference like elements.

FIG. 1 is a perspective view of a Lamb wave type high frequency deviceof a first embodiment of the invention.

FIG. 2 is a sectional view showing A-A cut surface in FIG. 1.

FIG. 3 is a graph showing a frequency temperature characteristic of theLamb wave type high frequency device of the first embodiment of theinvention.

FIG. 4 is a sectional view schematically showing a Lamb wave type highfrequency device of a second embodiment of the invention.

FIG. 5 is a perspective view schematically showing a Lamb wave type highfrequency device of a third embodiment of the invention.

FIG. 6 is a perspective view schematically showing a Lamb wave type highfrequency device of a fourth embodiment of the invention.

FIG. 7 is a perspective view schematically showing a Lamb wave type highfrequency device of a fifth embodiment of the invention.

FIG. 8 is a sectional view schematically showing a Lamb wave type highfrequency device of the fifth embodiment of the invention, which ishoused in a package.

FIGS. 9A to 9F are sectional views schematically showing main processesof manufacturing a Lamb wave type high frequency device, according to afirst method thereof in the invention.

FIGS. 10A and 10B are sectional views schematically showing a partprocess of manufacturing a Lamb wave type high frequency device of thefourth embodiment, according to a second method thereof in theinvention.

FIGS. 11A and 11B are sectional views schematically showing a partprocess of manufacturing a Lamb wave type high frequency device of thefirst embodiment, according to a third method thereof in the invention.

FIGS. 12A to 12D are sectional views schematically showing mainprocesses where SiO₂ is used as a sacrificed layer, according to afourth manufacturing method of the invention.

FIGS. 13A to 13C are sectional views schematically showing mainprocesses where a thermosetting resin is used as a sacrificed layer,according to a fifth manufacturing method of the invention.

FIGS. 14A and 14B show a sixth manufacturing method of the invention.

FIG. 14A is a perspective view showing a state after a sacrificed layeris formed.

FIG. 14B is a cross section showing C-C cut surface in FIG. 14A.

FIG. 15 is a sectional view showing a state where the sacrificed layermade of thermosetting resin is formed according to a seventhmanufacturing method of the invention.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

Embodiments of the invention will be described with reference to theaccompanying drawings.

FIG. 1 to FIG. 3 show a Lamb wave type high frequency device accordingto a first embodiment of the invention. FIG. 4, FIG. 5 and FIG. 6 showthe devices according to a second, third and fourth embodiments of theinvention, respectively. FIG. 7 and FIG. 8 show the device according toa fifth embodiment of the invention. FIG. 9A to FIG. 11B show methods ofmanufacturing a Lamb wave type high frequency device of the invention.

First Embodiment

FIGS. 1 and 2 show a Lamb wave type high frequency device according tothe first embodiment of the invention. FIG. 1 is a perspective viewthereof and FIG. 2 is a sectional view schematically showing the A-A cutsurface in FIG. 1. In FIG. 1, a Lamb wave type high frequency device 10comprises a piezoelectric substrate 20 and a reinforcing substrate 50that are connected each other.

The piezoelectric substrate 20 is a thin substrate made of quartz andhas a uniform thickness. The substrate 20 comprises an IDT electrode 30,which is made of aluminum and has a comb-teeth shaped, on one mainsurface of it and a pair of reflectors 41 and 42 in the both sides ofpropagating direction of a Lamb wave from the IDT electrode 30. The IDTelectrode 30 comprises an IN/OUT electrode 31 and a GND electrode 32.The IN/OUT electrode 31 is provided with a plurality of electrodefingers 31 a and a bus bar 31 b connecting the fingers 31 a each other.The GND electrode 32 is provided with a plurality of electrode fingers32 a and a bus bar 32 b connecting the fingers 32 a each other. Then,the electrode fingers 31 a and 32 a are interdigitated to form crossfinger electrodes.

The reflectors 41 and 42 are provided with electrode fingers 41 a and 42a that are arranged in a lattice, the bus bar 41 b connecting the bothends of the electrode fingers 41 a and the bus bar 42 b connecting theboth ends of the electrode fingers 42 a.

The Lamb wave type high frequency device 10 having the IDT electrode 30is called as a one-port resonator.

FIG. 2 shows the relationship among the IDT electrode 30, the reflectors41 and 42 and the reinforcing substrate 50. FIG. 1 also shows suchrelationship. In FIG. 2, the pitch between electrode fingers 31 a and 32a is P_(I), the line width of electrode fingers 31 a and 32 a is L_(I),pitches of IN/OUT electrode 31 and GND electrode 32 are λ, thewavelength of a Lamb wave, and the thickness of electrodes is H_(I).Here, λ, the wavelength of a Lamb wave, is a twice of the pitch P_(I).

Further, the pitch of electrode fingers 42 a of the reflector 42 isP_(r), the line width is L_(r) and the thickness is H_(r).

In the Lamb wave type high frequency device, it is known that a Lambwave is efficiently excited by setting the relationship between thethickness of H of a piezoelectric substrate 20 and λ, the wavelength ofa Lamb wave, within the range of 0<2H/λ≦10, which is disclosed inJP-A-2003-258596. As a result, the thickness of the piezoelectricsubstrate 20 is several μm to several tens μm.

The back surface of the piezoelectric substrate 20 is connected to thereinforcing substrate 50. The back surface is the other main surfaceopposite to one main surface on which the IDT electrode 30 and thereflectors 41 and 42 are formed. The reinforcing substrate 50, a platysubstrate made of Si, has a boxlike recess 53 formed in the center.Hence, the reinforcing substrate 50 comprises the recess 53 in thecenter and a rim portion 51 provided around all circumferences of therecess 53.

The piezoelectric substrate 20 is connected to a connecting surface 52,which is an upper surface of the rim portion 51 of the reinforcingsubstrate 50, by connecting means such as chemical bonding or adhesives,and includes a space portion 54. This space portion 54 has a vacuumedstate. Here, as shown in FIG. 1, the connecting surface 52 contacts thespace portion 54 at an inner circumference surface P, which is far fromthe IDT 30 and the reflectors 41 and 42. That is, the space portion 54forms an area wider than a region in which a Lamb wave is propagated.Therefore, propagating a Lamb wave is not affected at all by theconnection.

Further, the piezoelectric substrate 20 is reinforced since allcircumferences thereof are connected to the rim portion 51 of thereinforcing substrate 50.

An input drive signal having a given frequency is input to the IDTelectrode 30 so that the Lamb wave type high frequency device 10constituting the above components excites a Lamb wave. Then, the Lambwave is propagated to the direction of the reflectors 41 and 42 withbeing reflected on the back and front surfaces of the piezoelectricsubstrate 20. Then, the wave is reflected by the reflectors 41 and 42.

Next, a frequency temperature characteristic of the Lamb wave type highfrequency device 10 according to the embodiment is explained.

FIG. 3 is a graph showing a frequency temperature characteristic of theLamb wave type high frequency device 10. The vertical axis shows theresonance frequency deviation (ppm) while the horizontal axis showstemperature (° C.). As shown in FIG. 3, the frequency temperaturecharacteristic of the Lamb wave type high frequency device 10 using aquartz substrate as the piezoelectric substrate 20 is superior to thatof devices using an STW cut quartz substrate and an ST cut quartzsubstrate (ST-SAW).

Hence, according to the first embodiment, all circumferential areas ofthe recess 53 (the space portion 54) are connected to the reinforcingsubstrate 50, enhancing the structural strength, and holding the Lambwave type high frequency device 10 that is uneasily broken and easilyhandled, even if the thickness of the piezoelectric substrate 20 isseveral μm, which thickness is required as the exciting condition.

Further, the space portion 54, which has an area that is larger than aregion in which a Lamb wave is propagated, is provided between thepiezoelectric substrate 20 and the reinforcing substrate 50, and thespace portion 54 is vacuumed. The structure realizes the Lamb wave typehigh frequency device 10 that excludes energy loss of Lamb waveexcitation at the back surface of the piezoelectric substrate in amanner maintaining a structural strength, resulting in the stableresonance property having high excitation efficiency.

Further, using a quartz substrate as the piezoelectric substraterealizes a temperature frequency characteristic that is superior to thatof the STW cut quartz substrate and ST cut quartz substrate (ST-SAW).

Second Embodiment

The second embodiment of the invention is explained with reference to afigure. In the second embodiment, the recess is provided in thepiezoelectric substrate, though it is provided in the reinforcingsubstrate in the first embodiment shown in FIG. 2. The major differencefrom the first embodiment is explained.

FIG. 4 is a sectional view schematically showing the Lamb wave type highfrequency device 10 of the second embodiment. In FIG. 4, the IDTelectrode 30 and the reflectors 41 and 42 are foamed on the one surface(the front surface) of the piezoelectric substrate 20 and a boxlikerecess 23 is provided to the other surface (the back surface) of it.

The thickness H at the bottom of the recess 23 is set to the followingrelationship regarding the wavelength λ, of the Lamb wave: 0<2H/λ≦10. Arim portion 21 is formed at the circumferential area of the recess 23and a connecting surface 22 of the upper surface (the lower surface inthe figure) of the edge portion 21 is bonded to the reinforcingsubstrate 50 with chemical bonding or an adhesive, forming a spaceportion 24. The reinforcing substrate 50 is a thick substrate made ofSi.

Hence, according to the second embodiment, the structural strength ofthe piezoelectric substrate 20 is enhanced due to the rim portion 21bonded with the reinforcing substrate 50 as a reinforcing portion. Thespace portion 24 has a vacuumed state. Therefore, the second embodimenthas the same efficiency of the first embodiment.

Third Embodiment

The third embodiment of the invention is explained with reference to afigure. In the third embodiment, the groove-like recess is provided inthe reinforcing substrate, though the boxlike recess is provided in thereinforcing substrate in the first embodiment shown in FIG. 2. The majordifference from the first embodiment is explained.

FIG. 5 is a sectional view schematically showing a Lamb wave type highfrequency device 110 of the third embodiment.

In a reinforcing substrate 150, a groove-like recess 153 (an almostsquare C shape from the front view) is formed. The recess 153 is formedas crossing the reinforcing substrate 150, making the side surface ofthe width direction opened and a rim portion 151 is formed at both endsof the reinforcing substrate 150 in the longitudinal direction. Then,the piezoelectric substrate 20 is bonded to the reinforcing substrate150 at a connecting surface 152 of the upper surface of the rim portion151 by connecting means such as chemical bonding or an adhesive, forminga space portion 154. The piezoelectric substrate 20 has the samestructure of the piezoelectric substrate in the first embodiment shownin FIG. 2.

Here, the direction of forming the recess 153 is not limited as long asthe recess 153 has an area, which includes a region far from the IDTelectrode 30 and the reflectors 41 and 42, namely, which area is widerthan a region of a Lamb wave propagated.

Fourth Embodiment

The fourth embodiment of the invention is explained with reference to afigure. In the fourth embodiment, the groove-like recess is provided inthe piezoelectric substrate, though the boxlike recess is provided inthe piezoelectric substrate in order to form a space portion in thesecond embodiment shown in FIG. 4. The major difference from the secondembodiment is explained.

FIG. 6 is a sectional view schematically showing a Lamb wave type highfrequency device 210 of the fourth embodiment. In a piezoelectricsubstrate 120, a groove-like recess 123 (an almost square C shape fromthe front view) is formed. The recess 123 is formed as crossing thepiezoelectric substrate 120, making the side surface of the widthdirection opened and a rim portion 121 is formed at both ends of thepiezoelectric substrate 120 in the longitudinal direction. Then, thepiezoelectric substrate 120 is bonded to a reinforcing substrate 250 ata connecting surface 122 of the upper surface (the lower surface in thefigure) of the rim portion 121 by connecting means such as chemicalbonding or an adhesive, forming a space portion 124. The reinforcingsubstrate 250 has the same structure of the reinforcing substrate in thesecond embodiment shown in FIG. 4.

Here, the direction of forming the recess 123 is not limited as long asthe recess 123 has an area, which includes the region far from the IDTelectrode 30 and the reflectors 41 and 42, namely, which area is widerthan a region of a Lamb wave propagated.

According to the third and fourth embodiments, the structural strengthof the piezoelectric substrate 120 is enhanced in the similar manner ofthe first and second embodiments, though the recess 153 is formed in thereinforcing substrate 150 and the recess 123 is formed in thepiezoelectric substrate 120.

The front views of the recesses 123 and 153 are an almost square C shapein which pairs of opposed side surfaces of the piezoelectric substrate120 and the reinforcing substrate 150 are opened. A sacrificed layer isformed in the recesses 123 and 153, and then the sacrificed layer iseasily removed from the opening of the side surface after the IDTelectrode is formed. Details are described later.

Fifth Embodiment

The fifth embodiment of the invention is explained with reference to afigure. In the fifth embodiment, any of the Lamb wave type highfrequency devices 10, 110 and 210 shown in the first to fourthembodiments are encapsulated in a package. Here, the Lamb wave type highfrequency device 10 shown in the first embodiment is exemplified. Thesame reference numerals are applied to the same structures.

FIG. 7 is a sectional view schematically showing the Lamb wave type highfrequency device 10 of the fifth embodiment. In FIG. 7, there are aplurality of pads on the upper surfaces of bus bars 31 b, 32 b, 41 b and42 b of the IDT electrode 30 and the reflectors 41 and 42 formed on thesurface of the piezoelectric substrate 20.

More specifically, each two pads 33, totally four pads, are provided onthe bus bar 31 b, and the bus bar 32 b of the IDT electrode 30respectively. Further, each two pads 33, totally four pads, are providedon the bus bar 41 b, and the bus bar 42 b of the reflectors 41 and 42respectively. These pads 33 and 34 are made of metal such as Au andsolder, or conductive adhesives, and have an approximately halfspherical shape that is the same size.

As shown in FIG. 7, these pads 33 and 34 are arranged in a well-balancedmanner in a plan view in order that the Lamb wave type high frequencydevice 10 is housed in a package 60 shown in FIG. 8 and bonded. Hence,the allocation and the number of pads 33 and 34 are not limited.

The Lamb wave type high frequency device 10 including the pads 33 and 34is encapsulated in the package 60.

FIG. 8 is a cross section schematically showing the state in which theLamb wave type high frequency device 10 is encapsulated in the package60 and B-B cut section in FIG. 7. FIG. 7 is used as a reference. In FIG.8, the package 60 comprises a case 70 and a lid 80. The case 70 and thelid 80 are made of ceramics in the embodiment.

The case 70 includes a base 70 a and a rim portion 70 b, which arelaminated. Connecting electrodes 71 and 72, which are formed inside ofthe base 70 a, are extended at the connecting area of the base 70 a andthe rim portion 70 b. These electrodes are connected to the outerconnecting terminals 75 and 76 that are provided on the back surface ofthe base 70 a (connection is not shown in the figure).

The Lamb wave type high frequency device 10 is fixed so that thepiezoelectric substrate 20 faces toward the base 70 a. The pads 33provided in the bus bar 31 b are placed at the connecting electrode 71,the pads 33 provided in the bus bar 32 b are placed at the connectingelectrode 72, and the pads 34 provided in the bus bars 41 a and 42 b areplaced at the electrode lands 73 and 74. These pads are instantaneouslybonded by heating and adding pressure, for example. This bonding iscalled flip chip mounting.

Hence, the IN/OUT electrode 31 shown in FIG. 1 is connected to the outerconnecting terminal 75 and the GND electrode 32 shown in FIG. 1 isconnected to the outer connecting terminal 76, making it possible toreceive a given excited drive signal from outside.

Further, the reflectors 41 and 42 are electrically independent from theIDT electrode 30. The electrode lands 73 and 74 are provided so that theLamb wave type high frequency device 10 is hold with well balanced andfixed to the base 70 a with highly contact strength.

After the Lamb wave type high frequency device 10 is encapsulated intothe case 70, a lid 320 is bonded to the rim portion 70 b by connectingmeans such as chemical bonding or an adhesive. In this case, a spaceportion 61 within the package 60 is in a vacuumed state.

In the embodiment, the Lamb wave type high frequency device 10 accordingto the first embodiment is encapsulated into the package 60. But, it ispossible to encapsulate Lamb wave type high frequency devices accordingto the second embodiment to fourth embodiment into the package 60 in asimilar manner.

In the first and second embodiments, the space portions 54 and 24 are invacuumed states and the space portion 61 is also vacuumed within thepackage 60. Namely, all areas for exciting a Lamb wave are in a vacuumedstate.

Further, in the third and fourth embodiments, in case of packaging, whenthe space portion 61 is in a vacuumed state, the space portion 154 andthe space portion 124 of which a part is opened can be simultaneously ina vacuumed state.

Hence, according to the fifth embodiment, the Lamb wave type highfrequency device 10 reinforced by the reinforcing substrate 50 isencapsulated by the package 60, protecting the device 10 from the outerenvironments.

Further, it is well known that excitation property is remarkablydeteriorated when the IDT electrode 30 is hurt or attached with water ordust. However, in the above embodiments, it is encapsulated in thepackage 60 and held in a vacuum state, protecting the IDT electrode 30and maintaining good resonance property.

Further, the inside of the package is in a vacuum state, restrainingenergy loss at the IDT electrode 30.

Further, the pads 33 and 34 are provided on the bus bars 31 b, 32 b, 41b, and 42 b that do not affect excitation, and connected to theconnecting electrodes 71 and 72 and the electrode lands 73 and 74. Thisstructure electrically connects the connecting electrode 71 to the pads33 and connecting electrode 72 to the pads 34, and surely fixes the Lambwave type high frequency device 10 to the base 70 a of the package 60.

Using the flip chip mounting realizes both fixing and connecting thedevice simultaneously, downsizing the thickness and the area formounting, and making the device a low height and miniaturized comparedto wire bonding conventionally used for mounting surface acoustic waveelements.

First Manufacturing Method

Next, a method for manufacturing a Lamb wave type high frequency deviceof the invention is explained with reference to the drawings.

FIGS. 9A to 9F are sectional views schematically showing main processesof manufacturing a Lamb wave type high frequency device in theinvention. In FIGS. 9A to 9F, the Lamb wave type high frequency device110 in the third embodiment shown in FIG. 5 is exemplified. The device110 has the groove-like recess 153 in the reinforcing substrate 150.

First, the groove-like recess 153 is formed on the reinforcing substrate150, which is made of a silicon plate. As a method of forming the recess153, there are two methods such as removing a portion corresponding tothe recess 153 by etching, and depositing the rim portion 151 at theboth opposite ends of a plate base 155. Further, a grinding process isalso selected as the above method since the recess is like a groove.

Next, as shown in FIG. 9B, a sacrificed layer 156 made of zinc oxide(ZnO) is formed within the recess 153 by CVD or the like, and the uppersurface of the sacrificed layer 156 and the rim portion 151 are smoothedby CMP and the like.

As the sacrificed layer 156, a metal such as aluminum nitride (AlN), Al,Cu, Cr and Ag is used instead of zinc oxide. These materials for asacrificed layer are etched by an etchant, which is different from theone used for the piezoelectric substrate made of quartz. Namely, amaterial should be selected that the etchant for the material does notresolve the back surface of the piezoelectric substrate, when thesacrificed layer is etched and removed.

Further, as shown in FIG. 9C, a thick plate 20 a is connected to thereinforcing substrate 150 by connecting means such as chemical bondingor an adhesive. Here, the thick plate 20 a is a quartz substrate that isa material for forming a piezoelectric substrate, while the sacrificedlayer 156 is formed in the reinforcing substrate 150. Here, thethickness of the thick plate 20 a is 100 μm.

Next, as shown in FIG. 9D, the piezoelectric substrate 20 having apredetermined thickness is formed by polishing the thick plate 20 aunder the state where the thick plate 20 a is connected to thereinforcing substrate 150. The thickness H of the piezoelectricsubstrate 20 is set to the following range related to the wavelength ofthe Lamb wave: 0<2H/λ≦10. Specifically, it is several μm.

Next, the IDT electrode 30 and the reflectors 41 and 42 are formed onthe surface of the piezoelectric substrate 20 by photolithography asshown in FIG. 9E.

Then, as shown in FIG. 9F, the sacrificed layer 156 is removed byrelease etching so as to form the space portion 154.

Here, the IDT electrode 30 and the reflectors 41 and 42 may be formedafter release etching.

The pads 33 and 34 shown in FIG. 7 can be formed directly after the IDTelectrode 30 and the reflectors 41 and 42 are formed.

Second Manufacturing Method

Next, a second method for manufacturing the Lamb type high frequencydevice 210 in the fourth embodiment shown in FIG. 6 will be explainedreferring to FIGS. 10A and 10B. The major difference from the abovemethod is explained.

First, the groove-like recess 123 is formed in the thick plate 20 a of aquartz substrate by etching or the like and the sacrificed layer 125made of zinc oxide is formed within the recess 123 by CVD or the like.Then, the upper surfaces of the sacrificed layer 125 and the rim portion121 are smoothed by CMP or the like.

Then, the substrate 20 is bonded to the reinforcing substrate 150 byconnecting means such as chemical bonding or an adhesive. FIG. 10A showshow to be bonded.

Next, as shown in FIG. 10B, the piezoelectric substrate 120 having apredetermined thickness is formed by polishing the thick plate 20 a. Thethickness H of the piezoelectric substrate 120 is set to the followingrange related to the wavelength λ of the Lamb wave: 0<2H/λ≦10.Specifically, it is several μm.

Forming the IDT electrode 30 and the reflectors 412 and 42 and removingthe sacrificed layer 125 are the same manners and processes as alreadydescribed and shown in FIGS. 9E and 9F. Details are omitted.

According to the method for manufacturing a Lamb wave type highfrequency device, as shown in FIGS. 9A to 9F, the sacrificed layer 156is formed in the recess 153 of the reinforcing substrate 150 and thepiezoelectric substrate is bonded as it is thick, and polished to apredetermined thickness. Otherwise, as shown in FIGS. 10A and 10B, therecess 123 and the sacrificed layer 125 are formed under the state whenthe piezoelectric substrate is a thick plate 20 a, then after thesubstrate 20 a is bonded to the reinforcing substrate 150, the substrate20 is polished to a predetermined thickness. The sacrificed layers 156and 125 are removed to fowl the space portions 154 and 124 thereafter.

Hence, the sacrificed layers 156 and 125 are maintained until justbefore the Lamb wave type high frequency device is completed. Thisprocess can reduce the break of the piezoelectric substrate inmanufacturing processes, improving the yield of it.

Further, forming the recesses 153 and 123 like a groove easily removesthe sacrificed layers 156 and 125 from the opening of the both ends ofthe groove.

Third Manufacturing Method

A method for manufacturing the Lamb wave type high frequency deviceexplained in the first embodiment is explained with reference to thedrawings.

FIGS. 11A and 11B are sectional views schematically showing a partprocess of manufacturing the Lamb wave type high frequency device 10 ofthe first embodiment shown in FIGS. 1 and 2, according to a third methodthereof in the invention. The Lamb wave type high frequency device 10has the boxlike recess 53 in the reinforcing substrate 50, and there isno opened area at the state when the piezoelectric substrate 20 (thethick plate 20 a) is bonded to the reinforcing substrate 50. Hence, themanufacturing method shown in FIGS. 9A to 9F cannot remove thesacrificed layer 56. The third method includes forming a penetrationhole connecting the recess 53 to the bottom of the reinforcing substrate50 in the reinforcing substrate 50.

First, the penetration holes 57 and 58 connecting the recess 53 to thebottom of the reinforcing substrate 50 are formed in the reinforcingsubstrate 50. Then the sacrificed layer 56 at least filling the recess53 is formed as shown in FIG. 11A. Then, the surface of the reinforcingsubstrate 50 including the sacrificed layer 56 is smoothed by polishingand bonded to the thick plate 20 a of the piezoelectric substrate. Thefollowing processes are the same shown in FIGS. 9 C to 9F to completethe Lamb wave type high frequency device 10 shown in FIG. 11B.

Here, removing the sacrificed layer 56 uses the penetration holes 57 and58 provided in the reinforcing substrate 50. After removing thesacrificed layer 56, the space portion 54 is communicated with thepenetration holes 57 and 58 not shown).

In FIGS. 11A and 11B, two penetration holes 57 and 58 are provided. But,the number of holes is not limited to two. Further, the shape of apenetration hole is not limited, including a circle and a square.

Accordingly, the Lamb wave type high frequency device 10 can beencapsulated into the package 60 described in the fifth embodiment shownin FIG. 8 and the space portion 54 is maintained in vacuum.

The Lamb wave type high frequency device 10 described in the secondembodiment can also be formed by the above method. The Lamb wave typehigh frequency device 10 shown in FIG. 4 has the boxlike recess 23 inthe piezoelectric substrate 20 and there is no opened area at the statewhen the piezoelectric substrate 20 (the thick plate 20 a) is bonded.Here, not shown in the figure, it is possible to manufacture the deviceby providing a penetration hole communicating with the recess 23 of thepiezoelectric substrate 20 in the reinforcing substrate 50, and removingthe sacrificed layer from the penetration hole.

Hence, according to the above method, the sacrificed layer can beremoved using the penetration hole provided to communicate with therecess, though the structures of the first and second embodiments inwhich the boxlike recess is provided in the piezoelectric substrate 20or the reinforcing substrate 50.

Fourth Manufacturing Method

Next, a fourth method for manufacturing a Lamb wave type high frequencydevice of the invention is explained with reference to a figure. Thefourth method is for the Lamb wave type high frequency device having thesame structure of the Lamb wave type high frequency device 110 of thethird embodiment shown in FIG. 5, but the sacrificed layer is made ofSiO₂. The major difference from the third embodiment is explained. FIGS.9A to 9F, which show the method for manufacturing the Lamb wave typehigh frequency device of the third embodiment, are used as a reference.

FIGS. 12A to 12D are sectional views schematically showing mainprocesses where SiO₂ is used as a sacrificed layer, according to thefourth manufacturing method. First, as shown in FIG. 12A, a protectionlayer (may be called as an etching stopper layer) 25 against etching isentirely formed on the back surface of the thick plate 20 a made of aquartz substrate. The protection layer 25 is a thin metal layer, and ismade of a material, such as Au and Al, which is etched by an etchantdifferent from another etchant for quartz. The protection layer 25 isformed by vacuum evaporation, sputtering or the like.

Next, the sacrificed layer 156 made of SiO₂ is formed in the recess 153provided in the reinforcing substrate 150 by CVD or the like (see FIGS.9A and 9B). After the smoothing process by CMP, as shown in FIG. 12B,the thick plate 20 a, which is a quartz substrate on which theprotection layer 25 is formed, and the reinforcing substrate 150 inwhich the sacrificed layer 156 is formed are bonded, and then the thickplate 20 a is polished as the quartz substrate 20 having a predeterminedthickness.

Then, as shown in FIG. 12C, the IDT electrode 30 and reflectors 41 and42 are formed on the surface of the quartz substrate 20 byphotolithography. Next, a resist (not shown) is entirely coated on thesurface of the quartz substrate 20 including the IDT electrode 30 andthe reflectors 41 and 42, and then the sacrificed layer 156 is removedby etching.

The sacrificed layer 156 is removed using an etchant such as dilutehydrofluoric acid (DHF) and buffered hydrofluoric acid (BHF). In theetching, SiO₂ serving as a sacrificed layer is resolved and removed, butthe reinforcing substrate 150 made of Si, the surface of the quartzsubstrate 20 covered with the resist, and the protection layer 25 arenot dissolved. Then, the protection layer 25 is removed by etching. Theregion in which the protection layer 25 is removed may be wider than thearea of a Lamb wave propagated. That is, the region extends outside thearea in which the IDT electrode 30 and the reflectors 41 and 42 areformed.

After the resist is removed, the Lamb wave type high frequency device110 including the space portion 154 formed between the quartz substrate20 and the reinforcing substrate 150 is completed as shown in FIG. 12D.

Alternatively, the IDT electrode 30 and the reflectors 41 and 42 may beformed by removing the resist followed by removing the sacrificed layer156 and the protection layer 25.

In the above method, the Lamb wave type high frequency device 110 of thethird embodiment, in which the space portion 154 is formed by using thegroove-like recess 153, is exemplified. The method, however, can alsoapply to the Lamb wave type high frequency device 110, which includesthe boxlike recess 53, shown in FIGS. 1, 11A and 11B of the firstembodiment.

Further, the method can apply to the Lamb wave type high frequencydevice 210, in which the recess 153 is provided in the quartz substrate20, of the fourth embodiment shown in FIG. 6. In this case, theprotection layer against etching may be formed inside the recess 123.

Hence, according to the fourth manufacturing method, the quartzsubstrate 20 can be protected in etching the sacrificed layer byproviding the protection layer 25 on the back surface, which contactsthe sacrificed layer 156, of the quartz substrate 20, though SiO2, whichis typically used for a sacrificed layer of MESM structures, is employedfor the sacrificed layer.

Fifth Manufacturing Method

Next, a fifth method for manufacturing a Lamb wave type high frequencydevice of the invention is explained with reference to the drawings. Thefifth method is for the Lamb wave type high frequency device having thesame structure of the Lamb wave type high frequency device 110 of thethird embodiment shown in FIG. 5, but the sacrificed layer is made of athermosetting resin. The major difference from the third embodiment isexplained. FIGS. 9A to 9F, which show the method for manufacturing theLamb wave type high frequency device of the third embodiment, are alsoused as a reference.

FIGS. 13A to 13C are sectional views schematically showing mainprocesses where a thermosetting resin is used as a sacrificed layer,according to the fifth manufacturing method of the invention. First, asshown in FIG. 13A, the reinforcing substrate 150 including the recess153 and the thick plate 20 a of a quartz substrate are bonded to formthe space portion 154 between the reinforcing substrate 150 and thethick plate 20 a.

Next, the reinforcing substrate 150 and the thick plate 20 a areprovided into a setting jig 350 while they are bonded. FIG. 13B is across section (width direction) showing a cut surface of the Lamb wavetype high frequency device 110 shown in FIG. 5 along a directionperpendicular to the Lamb wave propagation direction. FIG. 13C is a planview of the Lamb wave type high frequency device 110. In FIGS. 13B and13C, the reinforcing substrate 150 is formed larger than the thick plate20 a in its width direction, resulting in the both sides of thereinforcing substrate 150 being projected in the width direction. Thesetting jig 350 surrounds the outer circumference of the reinforcingsubstrate 150 with a rim portion 350 a. The rim portion 350 a isprotruded over the upper surface of the reinforcing substrate 150.

In the setting jig 350, openings 153 a and 153 b are formed between thethick plate 20 a and the rim portion 350 a. The openings 153 a and 153 bare formed in an end face direction perpendicular to the direction(i.e., Lamb wave propagation direction) along which the IDT electrode 30and the reflectors 41 and 42 are formed in parallel. From the openings153 a and 153 b, a liquid thermosetting resin is injected into the spaceportion 154 to fill it. As a thermosetting resin, phenol resin, epoxyresin, urea resin, melanin resin, unsaturated polyester resin,polyurethane resin, thermosetting polyimide resin, or the like can beused. The thermosetting resin is cured by heating to form a sacrificedlayer 130.

In injecting the thermosetting resin, no bubbles remain inside the spaceportion 154 by injecting the resin from either one of the opening 153 aor the opening 153 b while the other one is suctioned.

After the sacrificed layer 130 is formed, the thick plate 20 a ispolished to a predetermined thickness. Next, the IDT electrode 30 andthe reflectors 41 and 42 are formed. Then, the sacrificed layer 130 isremoved to complete the Lamb wave type high frequency device 110.Processes after the polishing are as the same processes as shown inFIGS. 9C to 9F. The figures are omitted.

The sacrificed layer 130 can be removed by a solvent or the likeintroduced from the openings 153 a and 153 b. Therefore, the size of theopenings 153 a and 153 b may be set to any size from which athermosetting resin is easily injected and a solvent is easilyintroduced. The setting allows the Lamb wave type high frequency device110 to be easily taken out from the setting jig 350 after the sacrificedlayer 130 is removed.

In addition, the IDT electrode 30 and the reflectors 41 and 42 can beformed after the sacrificed layer 130 is removed. Thus, the IDTelectrode 30 and the reflectors 41 and 42 may be formed after taking outthe Lamb wave type high frequency device 110 followed by removing thesacrificed layer 130.

Hence, according to the fifth manufacturing method, using athermosetting resin to the sacrificed layer can inject and fill theresin in the space portion 154 after connecting the thick plate 20 awith the reinforcing substrate 150. This process does not need expensivemanufacturing facilities such as deposition apparatuses and chemicalvapor deposition (CVD) apparatuses.

In addition, the surface of the sacrificed layer 130 is flat and smoothsince the condition of the back surface of the quartz substrate 20 istransferred on it, resulting in a smoothing process such as CMP notbeing required. As a result, manufacturing processes can be shortened.Here, the openings 153 a and 153 b may be formed in the direction alongwhich the IDT electrode 30, and the reflectors 41 and 42 are formed inparallel.

Sixth Manufacturing Method.

Next, a sixth method for manufacturing a Lamb wave type high frequencydevice is explained with reference to the drawings. The method is amodification of the fifth manufacturing method. In the method, athermosetting resin is used for the sacrificed layer 130 and the settingjig is not used.

FIGS. 14A and 14B show the sixth manufacturing method. FIG. 14A is aperspective view showing a state after the sacrificed layer 130 isformed. FIG. 14B is a cross section showing C-C cut surface in FIG. 14A.In FIGS. 14A and 14B, the reinforcing substrate 150 includes a recess157.

The recess 157 is formed larger than the both sides of the thick plate20 a in the width direction. The periphery of the recess 157 issurrounded with 4 rim portions 151 a, 151 b, 151 c, and 151 d.Therefore, openings 155 a and 155 b are formed at the both sides of thethick plate 20 a in the width direction when the thick plate 20 a andthe reinforcing substrate 150 are bonded.

From the openings 155 a and 155 b, a liquid thermosetting resin isinjected into the space portion 154 to fill it. The thermosetting resinis cured by heating to form the sacrificed layer 130. After thesacrificed layer 130 is formed, the thick plate 20 a is polished to apredetermined thickness. Next, the IDT electrode and the reflectors areformed. Then, the sacrificed layer 130 is removed. As a result, the Lambwave type high frequency device 110 is completed.

The sacrificed layer 130 can be removed by a solvent introduced from theopenings 155 a and 155 b. Therefore, the size (width) of the openings155 a and 155 b may be set to any size from which a thermosetting resinis easily injected and a solvent is easily introduced.

Hence, according to the sixth manufacturing method, the same effect ofthe fifth manufacturing method can be achieved without the setting jigsince the thermosetting resin can be held in the recess 157 of thereinforcing substrate 150 with rim portions 151 a to 151 d when thethermosetting resin is injected into the space portion 154. Here, theopenings 153 a and 153 b may be formed in the direction along which theIDT electrode 30 and the reflectors 41 and 42 are formed in parallel.

Seventh Manufacturing Method

Next, a seventh method of manufacturing a Lamb wave type high frequencydevice is explained with reference to a figure. The method is amodification of the sixth manufacturing method. In the method, thereinforcing substrate includes a boxlike recess. The Lamb wave type highfrequency device has the same configuration as that has in the firstembodiment. The method is characterized in that a thermosetting resin isused for the sacrificed layer compared to the method shown in FIG. 11.Only the major difference from the sixth manufacturing method isexplained.

FIG. 15, which shows the seventh manufacturing method, is a sectionalview schematically showing a state in which the sacrificed layer 130made of a thermosetting resin is formed. In FIG. 15, the reinforcingsubstrate 50 includes the boxlike recess 53. The recess 53 forms thespace portion 54 closed by the rim portion 51 when the thick plate 20 aof a quartz substrate is bonded.

In the bottom surface of the recess 53, penetration holes 57 and 58communicating the inside of the space portion 54 with the outside of thespace portion 54 are provided in the reinforcing substrate 50. A liquidthermosetting resin is injected into the space portion 54 from eitherone of the penetration hole 57 or the penetration hole 58 while theother one is opened or suctioned. After the space portion 54 is filledwith the thermosetting resin, the resin is cured by heating to form thesacrificed layer 130. After the sacrificed layer 130 is formed, thethick plate 20 a is polished to a predetermined thickness, the IDTelectrode and the reflectors are formed, and the sacrificed layer 130 isremoved in order. As a result, the Lamb wave type high frequency deviceis completed.

Removing the sacrificed layer 130 uses penetration holes 57 and 58provided in the reinforcing substrate 50. After removing the sacrificedlayer 56, the space portion 54 is communicated with the penetrationholes 57 and 58 (not shown).

In FIG. 15, two penetration holes 57 and 58 are provided. But, thenumber of holes is not limited to two. Further, the shape of apenetration hole is not limited, including a circle and a square.

Hence, according to the seventh manufacturing method, the same effect ofthe fifth and sixth manufacturing methods is obtained and the settingjig used in the fifth manufacturing method is not required. In addition,rim portions, which hold the liquid thermosetting resin, shown in thesixth manufacturing method is not required since the planar shape of thequartz substrate 20 and the reinforcing substrate 50 coincide with eachother. As a result, the device can be miniaturized.

Further, the penetration holes 57 and 58 can be sealed after the spaceportion 54 is filled with a liquid or gas. As such, the reflectioncondition of a Lamb wave at the back surface can be changed by formingthe interface of the back surface of the quartz substrate 20 with aliquid or gas, enabling the options of resonance modes to be widened.

It should be understood that the invention is not limited to theabove-mentioned embodiments. Various modifications and improvements canbe made without departing from the spirit and scope of the invention.

For example, while the quartz substrate is exemplified as apiezoelectric substrate in the above embodiments, piezoelectricsubstrates such as lithium tantalite, lithium niobate, lithiumtetraborate, langasite, langanite, and kalium niobate, and othernon-piezoelectric substrates can be used.

In addition, piezoelectric thin films such as zinc oxide, aluminumnitride, and tantalum pentoxide, and piezoelectric semiconductors suchas cadmium sulfide, zinc sulfide, gallium arsenic, and indium antimonidecan be applied to a piezoelectric substrate according to the abovemanufacturing methods in which a sacrificed layer is formed.

Further, while the one-port resonator is exemplified as a Lamb wave typehigh frequency device in the above embodiments, a two-port resonator ora filter provided with an IDT electrode and reflectors can be applied toa Lamb wave type high frequency device.

Hence, according to the invention, a Lamb wave type high frequencydevice that has a high structural strength and can realize stablecharacteristics, and a manufacturing method with an improved yield inwhich the device is uneasily broken can be provided.

The entire disclosure of Japanese Patent Application Nos: 2006-039297,filed Feb. 16, 2006 and 2006-209507, filed Aug. 1, 2006 are expresslyincorporated by reference herein.

1. A method for manufacturing a Lamb wave type high frequency device that includes, a piezoelectric substrate, an interdigital transducer (IDT) electrode formed on a first main surface of the piezoelectric substrate, a reinforcing substrate connected to a second main surface of the piezoelectric substrate, a space portion formed in one of the piezoelectric substrate and the reinforcing substrate, an area of the space portion being larger than a region in which a Lamb wave is propagated, and a connecting surface formed in a periphery of the space portion, the method comprising: forming a groove-like recess corresponding to the space portion provided in one of a thick plate of the piezoelectric substrate and the reinforcing substrate; forming a sacrificed layer in the recess; connecting the thick plate with the reinforcing substrate; polishing the thick plate to a given thickness after the connection; and forming the IDT electrode and removing the sacrificed layer after the polishing.
 2. A method for manufacturing a Lamb wave type high frequency device that includes, a piezoelectric substrate, an interdigital transducer (IDT) electrode formed on a first main surface of the piezoelectric substrate; a reinforcing substrate connected to a second main surface of the piezoelectric substrate, a space portion formed in one of the piezoelectric substrate and the reinforcing substrate, an area of the space portion being larger than a region in which a Lamb wave is propagated, and a connecting surface formed in a periphery of the space portion, the method comprising: forming a boxlike recess corresponding to the space portion in the reinforcing substrate; forming a penetration hole in a bottom of the recess; forming a sacrificed layer in the recess; connecting a thick plate of the piezoelectric substrate with the reinforcing substrate; polishing the thick plate to a given thickness after the connection; and forming the IDT electrode and removing the sacrificed layer after the polishing.
 3. The method for manufacturing a Lamb wave type high frequency device according to claim 1, wherein the piezoelectric substrate is made of a quartz substrate, and the scarified layer is made of a material etched by an etchant that is different from another etchant etching the piezoelectric substrate.
 4. The method for manufacturing a Lamb wave type high frequency device according to claim 1 further comprising: forming a protection layer against etching on the second main surface of the piezoelectric substrate made of a quartz substrate; forming the scarified layer with SiO₂; and removing the protection layer in a range that is lager than an area of the Lamb wave propagated after removing the sacrificed layer.
 5. A method for manufacturing a Lamb wave type high frequency device that includes, a piezoelectric substrate, an interdigital transducer (IDT) electrode formed on a first main surface of the piezoelectric substrate, a reinforcing substrate connected to a second main surface of the piezoelectric substrate, a space portion formed in one of the piezoelectric substrate and the reinforcing substrate, an area of the space portion being larger than a region in which a Lamb wave is propagated, and a connecting surface formed in a periphery of the space portion, the method comprising: forming a recess corresponding to the space portion provided in one of a thick plate of the piezoelectric substrate and the reinforcing substrate; connecting the thick plate with the reinforcing substrate; filling a thermosetting resin in the space portion to be a sacrificed layer and curing the thermosetting resin after the connection; polishing the thick plate to a given thickness after the curing; and forming the IDT electrode and removing the sacrificed layer after the polishing. 